Wind-powered generator systems are well-known in the art and have been used for decades for generating electrical power from wind energy. While numerous configurations exist and have been used with varying degrees of success, most wind-powered generator systems utilize a rotor or impeller that is configured to rotate in a prevailing wind. An electrical generator connected to the rotor is rotated by the rotor and produces useful electrical power from the rotational movement of the rotor.
A commonly used rotor design or configuration resembles an aircraft propeller in that it comprises a plurality of long, slender turbine blades (typically three, although a greater or lesser number of blades may also be used) mounted to a hub. The hub is in turn mounted to a support structure or mainframe so that the hub and blades are free to rotate with respect to the mainframe. The hub is typically mounted to shaft which drives one or more electrical generators. In order to extract a meaningful amount of energy from the wind, it is usually necessary to provide the rotor with long blades. Consequently, the rotor and mainframe must be mounted on a high tower or pylon in order to provide sufficient clearance for the rotating rotor blades as well as to elevate the rotor blades above the turbulent air caused by terrain variations, buildings, and other obstructions on the ground. The mainframe is also usually pivotally mounted to the tower or pylon to allow the rotor to be directed into the prevailing wind.
There is a trend for wind generator systems to become increasingly larger. Unfortunately, however, the larger blades associated with larger wind generator systems are subjected to greater static and dynamic loads. As a result, it is very desirable, and often necessary, to test in advance a proposed blade design to ensure that it will be capable of withstanding the expected loads without structural failure. It is also important to evaluate the fatigue resistance of the blade design.
Generally speaking, wind turbine blades are tested by applying loads to the blade in various directions. For example, one type of load is applied in a direction perpendicular to the longitudinal or long axis of the blade, and is often referred to as a bending load, or as a flap load in the wind turbine field. Another type of load is also applied in a direction perpendicular to the longitudinal axis, but also perpendicular to the direction of the applied bending or flap load, in order to assess the structural properties of the blade in the transverse or rotational direction. Such loads are often referred to as transverse loads, or as lead-lag loads in the wind turbine field. The load applied to the blade in a given direction may be time-invariant or “static.” Alternatively, the load may be made to vary with time, in which case the load is often referred to as “cyclic.” Static loads are generally useful in evaluating the stiffness and ultimate strength of the blade, whereas cyclic loads are generally useful in evaluating the fatigue resistance of the blade.
Several different types of test apparatuses have been developed and are being used to apply loads to wind turbine blades. One type of test apparatus uses hydraulic actuators to apply the desired loads to the blade. This type of apparatus is advantageous in that it can be used to apply loads in any desired direction by simply mounting the hydraulic actuators at the desired positions on the blade and by orienting the actuators in the appropriate directions. Loads in more than one direction may be applied simultaneously with such apparatus, which often reduces the time required for testing. In addition, both static and cyclical loads may be applied by such apparatus.
Unfortunately, however, hydraulic testing systems of the type just described are not readily scalable, and it is difficult to use such an apparatus to test larger blades. For example, larger blades require larger deflections, thereby increasing the amount of hydraulic fluid that must be pumped to the actuators. While larger pumps can be used, there is a limit to the maximum pump size that can be practically used, both from a power requirement standpoint and from the standpoint of pump system cost. It is also difficult to provide actuators capable of producing the larger blade deflections. Even if such large-deflection actuators can be provided, larger blade deflections usually require more time to achieve a given number of load cycles.
Another system for placing loads on wind turbine blades uses a rotating eccentric mass to vibrate the blade along the longitudinal axis. Thus, a rotating mass system may be used to apply a cyclical bending or flap load to the blade. The system is designed so that the rotational speed of the mass vibrates the blade at about the resonance frequency of the blade in the longitudinal direction. Accordingly, such systems are often referred to as resonant test systems. The resonant vibration of the blade reduces the amount of energy required to apply the cyclical loads, thus is theoretically advantageous for testing larger blades. Unfortunately, however, the rotating mass also places axial loads on the blade which, at the forces required to maintain significant fatigue stresses in the longer blades, can become unacceptably large. Another problem with a rotating mass system is that it has proven difficult to simultaneously apply both bending and transverse loads to the blade. That is, while such a rotating mass system may be used to apply cyclic transverse or “lead-lag” loads to the blade by re-orienting the position of the rotating mass with respect to the blade, it is not generally practical to operate both types of rotating mass systems simultaneously. Instead, the usual practice is to perform the two tests (e.g., bending and transverse vibrational tests) at different times.